1. Field of the Invention
The present invention relates to an enhanced electromagnetic interference shield, and in particular, to an electromagnetic interference shield that significantly reduces electromagnetic interference emissions from an optical transceiver, for example.
2. Background Information
Many electrical devices, when operated, generate emissions that include electromagnetic radiation. When this electromagnetic radiation influences the proper functioning of another device, the result is known as electromagnetic interference (also known as EMI).
In order to ensure that electromagnetic radiation is emitted only at acceptable levels, i.e., to prevent electromagnetic interference, various standards have been developed. For example, both the United States and Canada have determined acceptable electromagnetic radiation emission limits for electrical devices operating at set frequencies. If the electrical device exceeds the determined acceptable emission limits, the sale or use of the electrical device may be prohibited.
Various shield devices are known that can be used to reduce emitted electromagnetic radiation. The conventional shields typically cover a substantial portion of the associated electrical device, and are usually formed of a metal that, when grounded, will attenuate the electromagnetic radiation. However, shields will typically have one or more apertures (openings) formed therein, to allow for the passage of electrical or fiber-optic cables, for example, which are used to couple the shielded electrical device to other associated components. These apertures may allow for the passage of emitted electromagnetic radiation (in the form of electromagnetic radiation waves), thus raising the possibility of electrical magnetic interference.
However, it is known that an aperture will attenuate electromagnetic radiation waves when the aperture is less than xc2xd of the wavelength xcex to be attenuated (i.e., length of aperture  less than xc2xdxcex). Moreover, the smaller the aperture, the greater the attenuation of the electromagnetic radiation waves. The attenuation of electromagnetic radiation waves due to passage through an aperture can be determined using the following formula:
S=20 log (xcex/2L), where
S=the shielding effectiveness of the aperture (in decibels);
xcex=the wavelength of the electromagnetic radiation; and
L=the maximum linear length of the aperture (in meters).
Moreover, the wavelength xcex can be determined by dividing the velocity of the electromagnetic radiation wave (i.e., the wave speed, which is approximately 3xc3x97108 m/sec2) by the frequency of the electromagnetic radiation emissions.
Thus, as the operational frequency (and hence, speed) of an electrical device increases, the associated wavelengths become smaller, thus requiring smaller apertures. However, the apertures are typically limited in minimum size to allow for the passage of associated cables, for example. If the size of the apertures are reduced too much, then passage of the associated cables therethrough may be prohibited. Therefore, it is clear that the size of the aperture, which typically has a minimum size, may limit the speed (i.e., bandwidth) of the associated electrical device.
As an example, optical transceivers use pulses of light to carry signals and transmit and receive data at very high speeds. Typically, the light pulses are converted into, or converted from, associated electrical signals using known circuitry. Such optical transceivers are often used in devices, such as computers, in which data must be transmitted at high rates of speed.
Optical transceivers may use light emitting diodes (LEDs) or lasers to transmit the light pulses, and a photodiode to receive light pulses. Typically, the photodiode is located adjacent to the LED or laser, to form a so-called duplex optical transceiver. Fiber-optic cables are then coupled to the respective LED or laser, and the photodiode, so that the light pulses can be transmitted to and from other optical transceivers, for example.
Further, optical transceivers typically have standard sizes and shapes, so that they may be readily incorporated into a computer system, for example, without modification. For instance, a typical duplex optical transceiver has a length of 39.12 millimeters, a width of 25.40 millimeters, and a thickness of 10.35 millimeters.
The optical transceivers are normally located on either input/output cards or port cards that are connected to an input/output card. In order to facilitate the connection of the fiber-optic cable to the optical transceiver, the transceiver is usually located on a periphery of the card.
Moreover, in a computer system, for example, the input/output card (with the optical transceiver attached thereto) is typically connected to a circuit board, for example a mother board. The assembly may then be positioned within a chassis, which is a frame fixed within a computer housing. The chassis serves to hold the assembly within the computer housing.
The LED or laser, and the photodiode are typically operated at a very rapid speed in order to transmit and receive data. For example, a typical high-speed optical transceiver may transmit or receive up to 622 megabits/second. In order to transmit, for example, data at this speed, the LED or laser must be repeatedly activated and deactivated 622 million times per second. This rapid switching action generates large amounts of electromagnetic radiation. Likewise, the rapid operation of the photodiode will generate a large amount of electromagnetic radiation.
As is known, it is only at harmonics of an operational frequency that potentially harmful electromagnetic radiation emissions occur. For example, the aforementioned high-speed optical transceiver, when transmitting or receiving 622 megabits per second, operates at a frequency of about 622 MHz. This operational frequency does not generally generate problematic electromagnetic interference. However, harmful emissions may be generated at the harmonics of this frequency, i.e., at 1.244 GHz, 1.866 GHz, 2.488 GHz, etc. Moreover, the third harmonic frequency is generally one of the more difficult frequencies to attenuate (i.e., at 2.488 GHz in the aforementioned scenario).
If the frequencies of the emitted electromagnetic radiation are not sufficiently attenuated, electromagnetic interference may affect the performance of other components, for example other input/output components, that are either located on the same card as the optical transceiver or that are located on other cards. The electromagnetic interference may cause other input/output components, for example, to either transmit or receive faulty data, thus affecting the operation of the overall system.
To prevent electromagnetic interference from having an adverse affect on other components, it is known to keep optical transceivers adequately spaced apart from other components. However, this disadvantageously limits the number of components that can be placed within a predetermined area. Moreover, more area is needed to accommodate a set number of components with a predetermined spacing therebetween. However, as is readily known, the trend in computer systems, for example, is to provide more functionality in less space. Thus, keeping the optical transceivers separated from other components may not be a practical solution.
Alternatively, it is also known to use a chassis of a computer system, for example, which has the card containing the optical transceiver mounted thereto, as a shield to block the electromagnetic radiation from permeating to other devices. However, the chassis typically only holds the periphery of the card, and will not prevent electromagnetic interference from affecting other components that may be located on the same card as the optical transceiver.
Moreover, because the chassis does not completely encapsulate the card containing the optical transceiver, the typical chassis is only able to serve as a shield when the electromagnetic radiation emissions are relatively low. For example, optical transceivers that have a speed in the range of 155 megabits/second do not emit large amounts of electromagnetic radiation. With these low-speed optical transceivers, a chassis may help reduce the level of electromagnetic radiation so as to fall within industry standards. However, as the speed of optical transceivers increases, for example to a speed of approximately 622 megabits/second, the amount of electromagnetic radiation produced increases to a level where the standard chassis provides inadequate shielding.
Additionally, modifying the typical chassis so as to completely surround the card holding the optical transceiver, in order to provide increased shielding, is problematic. The resulting configuration would disadvantageously make it more difficult to remove and replace the cards from the chassis.
It is also known to provide a card-mounted shield which covers the optical transceiver in order to reduce the amount of electromagnetic radiation that is emitted from the transceiver. The shield is typically attached to the card that the optical transceiver is fastened to, grounded in a known manner, and covers the body of the optical transceiver. However, the typical conventional optical transceiver shield, when properly positioned over a standard optical transceiver, is completely open at its end, to allow for the passage of a fiber-optic cable. However, it has been determined that a majority of the electromagnetic radiation emitted by optical transceivers is emitted at the end of the transceiver that transmits and receives the light and is attached to the external cable (i.e., the end that is not shielded by the typical shield). Thus, the known optical transceiver shield disadvantageously does not prevent a majority of the electromagnetic radiation from permeating to other components. Therefore, there is a need for an electromagnetic interference shield that attenuates the electromagnetic radiation passing from an end of an optical transceiver.
Moreover, it is conventional to extend the end of the optical transceiver that transmits and receives the light through an aperture in a so-called tailstock. A tailstock is a plate that provides physical support for the associated electrical device (for example, the optical transceiver), and which may provide for a limited amount of electromagnetic radiation shielding in some situations. However, the aperture in the tailstock must be sized to allow the end of the optical transceiver to pass therethrough. As such, the aperture in the tailstock does not attenuate any of the electromagnetic radiation that is emitted through the end of the optical transceiver. Thus, there is a need for an electromagnetic interference shield that provides an aperture of a size significantly reduced relative to a size of an end of an optical transceiver, so as to attenuate electromagnetic radiation emitted from the end of the optical transceiver to acceptable levels.
Additionally, the conventional card-mounted shield is not tailored to extend through the aperture in the tailstock. As such, the end of the optical transceiver is conventionally not provided with any shielding at its periphery. This may disadvantageously allow further electromagnetic radiation to be emitted into the surrounding environment. Thus, there is a need for an electromagnetic interference shield that is tailored to cover a periphery of an end of an optical transceiver, and which will still allow the end of the optical transceiver to fit through a standard size aperture in a tailstock.
Another known solution for reducing electromagnetic radiation is to provide circuitry within or coupled to the optical transceiver to limit the effects of the electromagnetic radiation. Such circuitry is often tailored to limit the amount of electromagnetic radiation that is produced by the optical transceiver, and to eliminate as much of the produced electromagnetic radiation as possible. However, this circuitry disadvantageously increases the design complexity of the optical transceiver. The increased design complexity will raise the cost of the device, as the additional circuitry will need to be designed, tested, and manufactured. Moreover, the complex design may result in an optical transceiver that is much larger in size. Additionally, the circuitry may not be successful in reducing the electromagnetic radiation below industry standards, thus necessitating additionally shielding using other known methods. Thus, there is a need for a shield that reduces electromagnetic radiation to acceptable limits, without requiring special circuitry.
It is, therefore, a principle object of this invention to provide an enhanced electromagnetic interference shield.
It is another object of the invention to provide an enhanced electromagnetic interference shield that solves the above mentioned problems.
These and other objects of the present invention are accomplished by the enhanced electromagnetic interference shield disclosed herein.
According to one aspect of the invention, the electromagnetic interference shield includes a hollow member, such as a pipe that has a central axis, and that is open at opposing ends. Further, the pipe preferably has a rectangular cross-sectional profile, and is defined by four contiguous, connected walls, each of which is arranged at a right angle to an adjoining wall. Although other arrangements are within the scope of the invention, this particular configuration advantageously conforms the electromagnetic interference shield to the shape of a standard optical transceiver, which likewise has a rectangular cross-sectional profile. Moreover, although the configuration of the electromagnetic interference shield has been defined with respect to an axis, this does not require the shield to have a symmetrical configuration. To the contrary, the shield may be unsymmetrical relative to its axis.
According to yet a further aspect of the present invention, the electromagnetic interference shield advantageously slips over the end of the optical transceiver, so as to totally encapsulate the end periphery of the optical transceiver. Alternatively, it is also contemplated that the electromagnetic interference shield may be slipped within the end of the optical transceiver. Thus, the electromagnetic interference shield is easily and quickly positionable in its proper location.
Moreover, the electromagnetic interference shield advantageously shields the end region of the optical transceiver that is typically not covered using a conventional shield. Thus, the electromagnetic interference shield according to the present invention, when used in conjunction with a conventional shield, significantly increases the shielding coverage, as compared to the shielding effect provided using only a conventional shield.
According to a further exemplary embodiment of the invention, the electromagnetic interference shield is advantageously provided with a septum which extends between opposing sides of the pipe (i.e., arranged to extend in a direction that is at an angle to the axis of the pipe), and which is preferably centrally positioned. Thus, the septum advantageously effectively reduces the maximum size of the opening in the pipe by one-half. Moreover, the septum is preferably positioned so that when the electromagnetic interference shield is properly positioned relative to the optical transceiver, the septum will be located between a light-receiver and a light-emitter of the optical transceiver. Thus, the septum will significantly attenuate electromagnetic radiation waves (by reducing the maximum dimension of the aperture through which the electromagnetic radiation waves must pass), without hindering the connection of associated fiber-optic cables.
According to another exemplary aspect of the invention, the septum is disposed at an extreme end of the pipe. Thus, the placement of the septum will not interfere with the positioning of the pipe over the end of the optical transceiver. Moreover, the septum can serve as a mechanical stop, to ensure that the pipe is properly positioned relative to the end of the optical transceiver.
In a further exemplary aspect of the invention, the electromagnetic interference shield is provided with a number of conductive grounding springs, which are disposed around the outer periphery of the ends of the pipe. The grounding springs advantageously conductively couple the electromagnetic interference shield to a ground potential, and help to hold the electromagnetic interference shield in a fixed position, for example, by engaging with a conventional shield. Moreover, since it is conventional to fix the conventional shield to the circuit board on which the optical transceiver is disposed, the grounding springs likewise hold the electromagnetic interference shield in a predetermined position relative to the circuit board.
Additionally, when properly positioned, the grounding springs will engage with an inner edge of an aperture of a tailstock, thus advantageously holding the electromagnetic interference shield in a fixed position relative to the tailstock. Thus, the electromagnetic interference shield according to the present invention advantageously ensures that the conventional shield, the optical transceiver, and the aperture of the tailstock are properly aligned and positioned relative to one another.
The grounding springs may be integrally formed with the pipe. Thus, fewer separate components are necessary.